Antibodies, and various engineered forms thereof, are effective therapeutic agents currently being used to treat patients suffering from a variety of disorders. Some of these antibodies recognize antigens present on the surface of tumor cells. Cripto is a 188-amino-acid cell surface protein overexpressed by many tumor cells. Cripto was isolated in a cDNA screen of a human embryonic carcinoma library (Ciccodicola et al., 1989, EMBO J. 8:1987-91). Cripto was originally classified as a member of the EGF family (Ciccodicola et al., supra); however, subsequent analysis showed that Cripto did not bind any of the known EGF receptors and its EGF-like domain was actually divergent from the EGF family (Bianco et al., 1999, J. Biol. Chem. 274:8624-29).
Overexpression of the Cripto protein is associated with tumors in many tissues (including, but not limited to brain, breast, testicular, colon, lung, ovary, bladder, uterine, cervical, pancreatic and stomach). Panico et al., 1996, Int. J. Cancer 65:51-56; Byrne et al., 1998, J. Pathology 185:108-11; De Angelis et al., 1999, Int. J. Oncology 14:437-40.
Murine antibodies that bind to Cripto have been described. However, while murine antibodies do have applicability as therapeutic agents in humans, because they are not of human origin they may be immunogenic. Administration of such antibodies may result in a neutralizing antibody response (human anti-murine antibody (HAMA) response), which is particularly problematic if the antibodies are desired to be administered repeatedly, e.g., in treatment of a chronic or recurrent disease condition. Also, because they contain murine constant domains they may not exhibit human effector functions.
In an effort to alleviate the immunogenicity concerns, “humanized” antibodies are often produced. In one protocol, CDRs from an antibody of mouse origin are transferred onto human framework regions resulting in a “CDR grafted” antibody. Frequently, amino acid residues which could potentially affect antigen binding in the framework region are backmuated the corresponding mouse residue.
However, while humanized antibodies are desirable because of their potential low immunogenicity in humans, their production is unpredictable. For example, sequence modification of antibodies may result in substantial or even total loss of antigen binding affinity, or loss of binding specificity. In addition, despite sequence modification “humanized antibodies” may still exhibit immunogenicity in humans. The development of humanized anti-Cripto antibodies would be of great benefit in inhibiting the consequences of Cripto expression in the cells of patients. In addition, the development of such antibodies would provide a means for targeting Cripto positive tumor cells in order to deliver anti-tumor agents, such as toxins, radiolabels, and the like.